The present invention relates to an apparatus for constructing cylindrical storage tanks from coiled strip steel.
Large diameter cylindrical storage tanks are constructed at chemical plants, pulp mills, municipal water or oil related plants for the storage of products. Conventionally these tanks are fabricated on site from a large number of relatively small pre-formed curved heavy gauge steel plates. The steel plates are slung and placed by a crane or other lifting device and are then tediously hand fitted, tack-welded and finally welded in place. Scaffolding must be erected around the tank to allow the workers to work at the height of the tank under construction. This method of tank construction is hazardous, due to the heights involved, time consuming, due to the tedious hand-fitting involved, and expensive, due to the need for accurately pre-formed, curved steel plates and a large work force.
Attempts have been made to construct these steel tanks directly from coiled steel in order to eliminate the need for the expensive pre-formed steel plates. In U.S. Pat. No. 3,380,147 issued on Apr. 30, 1968, to McDonald, a method of constructing steel tanks from strip steel is disclosed. Strip steel is fed from a coil of steel mounted on a stationary turntable, through a plurality of stationary crimping rollers onto a helically arranged support structure. Thus the steel is fed in a spiraling manner onto the lower tank walls, as the upper, completed tank walls are progressively elevated. The crimping rollers bend the lower edge of the strip steel in the tank wall such that it overlaps the upper edge of the strip steel in the wall therebelow.
In U.S. Pat. No. 4,121,747 issued on Oct. 24, 1978, to McFalter, and in U.S. Pat. No. 4,142,284 issued to Steuber, similar methods are disclosed for constructing helically wound steel tanks from coiled strip steel. Here again the strip steel is pulled from a coil mounted on a stationary turntable, through stationary pinch rollers to provide the proper curvature, through a stationary strip alignment assembly to align the strip with the tank wall thereabove, and onto a helically arranged support structure.
While the above methods may eliminate some of the problems associated with the use of pre-formed steel plates, it appears that they too have drawbacks. Firstly, the upper and lower finished edges of the helically wound tank are uneven. Thus to attach a roof or a floor, these edges must usually be cut off. Secondly, the abovementioned procedures require the rotation of the entire tank as it is assembled. In addition to the energy required to do this, a very stable and complex support system is needed. Further, the use of a stationary turntable for the coil steel, the stationary pinching rollers and the stationary strip alignment assembly, in my view, would require the use of accurately cut and wound coiled steel. While such steel may be available, it would be extremely expensive. In my experience, the coiled steel received from the steel mills is often off specification having uneven strip edges and curvature. Steel having such defects, when used in the prior art devices described, would require constant adjustments of the distantly spaced assemblies in order to feed the steel and fit the steel with the tank wall existing thereabove.
Further, these helically wound systems are limited to handling lightweight steel, for example in the construction of small storage bins for grain and the like. Heavy plate steel cannot be bent by crimping or corrugating operations described above. Also, in large cylindrical tanks the tank wall thickness often needs to be increased in the lower portions of the tank. Helically wound tanks cannot accommodate this change in wall thickness.